In a known mechanism, each word of the slave store comprises a field of equal size to a main store word and a so-called home address field of sufficient size to contain a main store address. As each word is transferred from the main store to the slave store, its address in the main store is written into the corresponding home address field. When an item of information is required, the home address fields of the slave store are associatively searched for the main store address of the required item. If a match is found, the required item of information is obtained from the slave store. If no match is found, the required item is transferred from the main store to the slave store, a previously entered word being first discarded from the slave store to make room for the word to be entered if necessary. The record of the information in the main store is left undisturbed.
In another known system, the use of an associative store is avoided by transferring only instructions to a slave store which is organised as a stack. In a further known mechanism, in which both data and instructions are transferred to a slave store which does not use associative addressing, a field of low order bits from the main address determines the slave store location to which information may be transferred. This restriction can cause information to be discarded from the slave store while the latter is only partially full.